Author Topic: Which fin on your DW board?  (Read 15605 times)

Eagle

  • Cortez Bank Status
  • *****
  • Posts: 2426
    • View Profile
Re: Which fin on your DW board?
« Reply #30 on: August 30, 2016, 01:02:24 PM »
For a double foiled fin, any angle of attack other than straight on generates lift that corrects the skew. The lift is perpendicular to the fin, and as soon as the board is going straight it goes away--or more accurately is balanced by the flow on the opposite side of a fin.....

.....But talking about lift raising the tail of the board or reducing drag is a little nutty.....

.....I've seen a lot of strange force diagrams, and most of them are baloney. Treating a SUP board in isolation, on flatwater, with only a paddle for propulsion there is only one propulsive force being applied--that from the paddle--and it is being applied off-center to the board. If there were no fin the resultant force vector  would turn the board in circles and only the rail and board drag would resist that movement. So it's valid to talk about fin efficiency in resisting that vector and optimizing the propulsive force as forward movement--and that's all. There's no magic force being generated by the fin. If there were you could give your board a gentle shove and watch it accelerate away.

These comments seem reasonable.  What we do is simply use a GPS to measure time distance and effort to ascertain benefit and efficiency.  Applies to board paddle and fin combinations.  Has always been accurate enough for us.
Fast is FUN!   8)
Dominator - Touring Pintail - Bullet V2 - M14 - AS23

pdxmike

  • Cortez Bank Status
  • *****
  • Posts: 6186
    • View Profile
Re: Which fin on your DW board?
« Reply #31 on: August 30, 2016, 02:08:48 PM »
Larry and Area 10,
Someone had written something here.
keith are your fins available in aluminum?
« Last Edit: August 30, 2016, 05:03:44 PM by Admin »

Area 10

  • Cortez Bank Status
  • *****
  • Posts: 4057
    • View Profile
Re: Which fin on your DW board?
« Reply #32 on: August 30, 2016, 02:17:57 PM »
pdxmike - that made me laugh out loud.

zachhandler

  • Malibu Status
  • **
  • Posts: 99
    • View Profile
    • Email
Re: Which fin on your DW board?
« Reply #33 on: August 30, 2016, 02:56:23 PM »
PonoBill - I have been thinking about your explanation of why fins are foiled - If I understand you correct, you are saying that it allows the fin to draw the tail back in line with the bow (via principle of lift) if the board has started to move in a diagonal manner through the water.

My confusion is this - you really don't need a foil shape to do that.  A flat fin cut out of a sheet metal would also direct the tail of the board back in line with the bow. How a fin works is more about the angle of attack than the foil shape I believe, similar to the more accurate explanation for how an airplane wing works and how airplanes can fly upside down:

http://www.telegraph.co.uk/news/science/science-news/9035708/Cambridge-scientist-debunks-flying-myth.html

In surfskis, a rudder without foil works, but it can only turn so far before it generates a great deal of turbulance that impedes deflection of boat relative to the water. Isn't that the main reason sup fins are foiled? to allow them to continue to be effective without turbulent "stalling" at a greater angle of attack?

Maybe I misread your post.  Anyway you know and think a lot about this stuff so I would be curious to get your take on it.


PonoBill

  • Cortez Bank Status
  • *****
  • Posts: 25864
    • View Profile
Re: Which fin on your DW board?
« Reply #34 on: August 30, 2016, 03:52:16 PM »
Completely true, but a foiled fin is more efficient about creating the required lift. An unfoiled plate stalls with a very low angle of attack and has turbulence with even a slight angle.

that video is kind of stupid. First of all, why is the wing in stall? But more importantly--the idea that the air has to reach the end of the wing at the same time is a strawman--I don't know who claims that, but it's nothing I've heard before. Obviously at some point the air becomes undisturbed again, but pressure differences persist after a wing has passed through any volume of air. Obviously there's no force mating up the flow, if there were, there would be no turbulence.  The air on top of the wing moves faster and further, and therefore is at a lower pressure. An easily measurable effect and easy to understand at the molecular level for a gas, a little harder to understand for an incompressible liquid.

High velocity airfoils are almost always symmetrical and thin--they don't need a big difference in velocity to generate lift, a small change in angle of attack is sufficient. If they had a similar asymmetric shape to lower velocity foils the turbulence and drag they generate would be much higher.
Foote 10'4X34", SIC 17.5 V1 hollow and an EPS one in Hood River. Foote 9'0" x 31", L41 8'8", 18' Speedboard, etc. etc.

yugi

  • Cortez Bank Status
  • *****
  • Posts: 2228
    • View Profile
Re: Which fin on your DW board?
« Reply #35 on: August 31, 2016, 04:51:13 AM »
...
A flat fin cut out of a sheet metal would also direct the tail of the board back in line with the bow. How a fin works is more about the angle of attack than the foil shape I believe
...

true

Completely true, but a foiled fin is more efficient about creating the required lift. An unfoiled plate stalls with a very low angle of attack and has turbulence with even a slight angle.
...

and also true.

However the shape and the surface area make a much greater difference overall. Profile would, of course, optimize for a given shape but that pales in comparison to pure shape and surface area.

At least that's what the hydrodynamics gurus at the MIT tunnel testing lab insisted on teaching me.

NB By flat profile I do still presume a rounded leading edge and tapered trailing edge .

SUP fins are so low thickness (compared to surface area) that  profile is very minimal as compared to a flat plate anyway. Plus in SUP we are rarely reaching limits of fin hold. We don't hang out after DW sessions talking about our spin outs very much, do we?

zachhandler

  • Malibu Status
  • **
  • Posts: 99
    • View Profile
    • Email
Re: Which fin on your DW board?
« Reply #36 on: August 31, 2016, 06:34:14 AM »
Yugi that is my impression as well. On a ski or oc you can mash the pedal and instantly achieve a huge angle of attack of the rudder; enough to disrupt laminar flow and turn a a steering device into a braking device. Seems much harder to do this on a board with a fixed fin because the angle of attack can only change at the rate that the direction of travel changes, which is much more gradual.

yugi

  • Cortez Bank Status
  • *****
  • Posts: 2228
    • View Profile
Re: Which fin on your DW board?
« Reply #37 on: August 31, 2016, 07:04:35 AM »
Exactly. IMO.

That said, as a sailor, I will always be impressed with, and gravitate to, a sweet profile.

I should be saying that it’s shape and surface area that I’ll be deciding on. But to tell you the truth, recently it’s within the click system. I’ve had the LA, Kumano, Futures little ball bearing click-ins. Most recently the Futures 9” Dolphin. Which I like. Now I have the new FCS click (Kalama), which does hold firmer, so I like more.

Even worse!!! I would have taken the Slater Trout FCS click fin if I was deciding on pure shape and surface area, but I don't like this years color and the Kalama fin colors really fit my board!

All that from a guy who has actually done tunnel testing for  keels (in reduced sizes, just about exactly our fin sizes BTW).

<duck>

Thereby proving the point that fin preferences really are personal.

Edit to add: I would love to try the Larry Alison/Jeremy Rigs Aercore fin. I like the little ball bearing click system, but the new FCS click system is firmer. Providing more peace of mind for DWs.
« Last Edit: August 31, 2016, 07:34:27 AM by yugi »

coldsup

  • Teahupoo Status
  • ******
  • Posts: 1430
    • View Profile
Re: Which fin on your DW board?
« Reply #38 on: August 31, 2016, 07:55:22 AM »
Funny - I have one of the Click FCS fins - just the cheapy SUP touring/Surf one but the shape looks pretty good and it works pretty good.....but on a DW I want a screw/plate in there.

Just a mind thing - I would probably be the first person ever to hit a fin the wrong way on a fall and lose one - 1 mile out to sea  ;D

yugi

  • Cortez Bank Status
  • *****
  • Posts: 2228
    • View Profile
Re: Which fin on your DW board?
« Reply #39 on: August 31, 2016, 09:03:56 AM »
^^ I used to do just that. But now with the new FCS click I do use on DWs.

PonoBill

  • Cortez Bank Status
  • *****
  • Posts: 25864
    • View Profile
Re: Which fin on your DW board?
« Reply #40 on: August 31, 2016, 09:05:28 AM »
Certainly surface area and profile can be overwhelming factors, but not all the time. For the 99 percent of the time that the surface area and profile is parallel to flow it does almost nothing other than producing lots of skin drag--there's no force applied. In that circumstance, most of the directional stability comes from lift.  A small change in angle of attack, that would expose a small fraction of the surface area, delivers a relatively large amount of lift. Most fin companies were trying high aspect, low area fins recently, because drag is low while directional stability is high for small corrections. But when the going gets a little rough, and the board is swinging more, the low aspect, high surface area fin works better.

You don't need much thickness for a foil to produce lots of lift in water. Take a look at any hydrofoil. I measured the thickness of some of the foils being used for downwinding--most are around .25". With a wing less than 2' wide they're lifting 200 pounds at 5mph. I calculated the list of the Geezer foil, formed from the wing of a glider that lifts perhaps five pounds in air with 5 feet of span. The section from the wingtip to the dihedral curve--about 13 inches--will lift about 170 pounds per wing at 5 mph. 340 pounds for the two wings at expected ride height. At 10mph I calculate that only eight inches of each wing will be submerged.

« Last Edit: August 31, 2016, 09:21:04 AM by PonoBill »
Foote 10'4X34", SIC 17.5 V1 hollow and an EPS one in Hood River. Foote 9'0" x 31", L41 8'8", 18' Speedboard, etc. etc.

yugi

  • Cortez Bank Status
  • *****
  • Posts: 2228
    • View Profile
Re: Which fin on your DW board?
« Reply #41 on: August 31, 2016, 09:57:14 AM »
I hear you and I know I need to try get used to a low-aspect larger surface. Rough is the way I like it.

I’m used to the high aspect FCS Kalama now.


I took an FCS Danny Ching for a DW and didn’t get in the groove with it.


I missed the anchored tail feel of the deep fin, I'm used to turning off the tail. Also I was a bit put off by the augmented roll of my board, even though I’m comfy as hell on it.

I’d like to try more with a low fin see if I get used to it and it reduces the dreaded death roll (broach). Which I don’t get much anyway.

Right now I’m jonesing for a 25” wide DW board and admit to getting the Kalama partly in anticipation of that. Knowing I could use the extra anti-roll stability of a deep fin to help my transition to a narrow board. Other than it looks cool…  I’ve always had a thing for the wing look of a Hatchet type fin.

Who was it who said recently: “an efficient fin would look like a modern airplane wing”?

OK, OK, OK, next thing someone is going to say is “what about the winglets on the tips”. Yes, I was there in Newport when Australia II’s keel was unveiled after winning the America’s cup. No, I’m going to be a grumpy old classic dude and stick with simple stuff for my SUPing. If I wanna get fancy I’ll buy a  foil for windsurfing. Kiting I think I’ll keep simple for travelling light.

Oooops, I digressed. Where were we? Oh yeah, fins are so personal.
« Last Edit: August 31, 2016, 10:28:36 AM by yugi »

PonoBill

  • Cortez Bank Status
  • *****
  • Posts: 25864
    • View Profile
Re: Which fin on your DW board?
« Reply #42 on: August 31, 2016, 06:07:41 PM »

Who was it who said recently: “an efficient fin would look like a modern airplane wing”?


I would say it wasn't me, though with my memory it could have been, and after a few beers I'll type silly shit like that. Even though the speed range we operate in is narrow, the stuff we ask fins to do demands a lot of fiddling. If there's any universal fin shape it's probably a dolphin. Airplane wings work hard to be as efficient as bird wings, fins work hard to be as good as dolphins. Birds and dolphins don't have a lot in common.
Foote 10'4X34", SIC 17.5 V1 hollow and an EPS one in Hood River. Foote 9'0" x 31", L41 8'8", 18' Speedboard, etc. etc.

iDownwind

  • Malibu Status
  • **
  • Posts: 56
    • View Profile
    • Email
Re: Which fin on your DW board?
« Reply #43 on: September 03, 2016, 05:17:42 AM »
Can you guys recommend a fin for my NSP DC Open Ocean 14x28? Is there a fin that will give the board more stability in the sometimes confused chop that we get with our DW runs on Lake Michigan? I don't mind giving up some maneuverability because the NSP seems to be super sensitive to foot steering. I really like the board over all, it's reasonably fast in the flats, seems to catch the tiniest of bumps with ease.  It has so much more volume than my V2 with A.S.S as well as my SIC F14. But when it gets big, i struggle a bit more than I'd like. If i can buy a bit more stability that would be great.

Thanks
SIC F14
SIC Bullet  V2 (A.S.S.)
SIC Recon 11.11
NSP Open Ocean 14'x28"

PonoBill

  • Cortez Bank Status
  • *****
  • Posts: 25864
    • View Profile
Re: Which fin on your DW board?
« Reply #44 on: September 03, 2016, 06:03:08 AM »
I'm pretty fond of Larry's downwind fins. They've performed the best for me all around. But a big, meaty dolphin fin adds a lot of stability without creating many new problems. I have a huge fiberglass Wingnut dolphin fin that's probably 10.5 or 11"  that does great things for my little 12'2". I foolishly changed it out for the Gorge Paddle Challenge which turned out to be so chopped up by all the competitors that I couldn't stay on the board. I was cursing myself. If I were looking just to add stability that's what I'd choose.
Foote 10'4X34", SIC 17.5 V1 hollow and an EPS one in Hood River. Foote 9'0" x 31", L41 8'8", 18' Speedboard, etc. etc.

 


SimplePortal 2.3.7 © 2008-2024, SimplePortal